An organic electroluminescence (EL) device is generally composed of an anode, a cathode, and one or more organic thin film layers sandwiched between the anode and the cathode. When a voltage is applied between the electrodes, electrons are injected from the cathode and holes are injected from the anode into a light emitting region. The injected electrons recombine with the injected holes in the light emitting region to form excited states. When the excited states return to the ground state, the energy is released as light.
Many researches have been made on the applications of organic EL device to display, etc. because of its possibility of a wide selection of emission colors by using various emitting materials in a light emitting layer. Particularly, the research on the materials which emit three primary red, green, and blue colors has been made most actively, and the intensive research has been made to improve their properties.
One of the most important problems involved in organic EL devices is the improvement of the emission efficiency. To obtain an organic EL device with high emission efficiency, it has been known to form a light emitting layer by doping a host material with a several percent of a dopant material. The host material is required to have a high carrier mobility, a uniform film-forming property, etc. The dopant material is required to have a high fluorescent quantum yield, a uniform dispersibility, etc.
Patent Literatures 1, 2 and 3 describe benzofluorene compounds as the materials for a light emitting layer.